Abstract
BRAF inhibitor (BRAFi) resistance compromises long-term survivorship of patients with malignant melanoma, and mutant NRAS is a major mediator of BRAFi resistance. In this study, employing phenotypic and transcriptomic analysis of isogenic melanoma cells that differ only by NRAS mutational status (BRAFi-sensitive A375-BRAFV600E/NRASQ61 vs. BRAFi-resistant A375-BRAFV600E/NRASQ61K), we show that BRAFi (vemurafenib) treatment selectively targets BRAFV600E/NRASQ61K cells upregulating epithelial-to-mesenchymal transition (EMT) gene expression, paradoxically promoting invasiveness and metastasis in vitro and in vivo. First, NanoString nCounter transcriptomic analysis identified the upregulation of specific gene expression networks (EMT and EMT to metastasis) as a function of NRASQ61K status. Strikingly, BRAFi treatment further exacerbated the upregulation of genes promoting EMT in BRAFV600E/NRASQ61K cells (with opposing downregulation of EMT-driver genes in the BRAFV600E/NRASQ61 genotype) as detected by EMT-focused RT2 Profiler qPCR array analysis. In BRAFV600E/NRASQ61K cells, BRAFi treatment enhanced proliferation and invasiveness, together with activation of phosphorylated protein kinase B (Ser473), with opposing phenotypic effects observable in BRAFV600E/NRASQ61 cells displaying downregulation of phosphorylated protein kinase B and phosphorylated extracellular signal-regulated kinase 1/2. In a SCID mouse bioluminescent melanoma metastasis model, BRAFi treatment enhanced lung tumor burden imposed by BRAFV600E/NRASQ61K cells while blocking BRAFV600E/NRASQ61 metastasis. These preclinical data document the BRAFi-driven enhancement of tumorigenesis and metastasis in BRAFi-resistant human BRAFV600E/NRASQ61K melanoma, a finding with potential clinical implications for patients with NRAS-driven BRAFi-resistant tumors receiving BRAFi treatment.